1. Field of the Invention
The invention relates to the field of data communications and finds particular application as a high speed, full duplex, modem.
2. Description of the Prior Art
While at one time telephone lines only carried voice communications, for many years now it has been common for digital data to be conveyed over the telephone network. However, this development has not been without significant technical limitations and obstacles. For example, the bandwidth of a typical "dial up" telephone line is only approximately 3 kilohertz (kHz) which serves as an upper limit to the data transfer rate upon the line. Further, impairments or performance limitations of the dial up telephone network have severely limited the ability of the network to reliably transfer digital data at high speeds. For example:
Telephone lines suffer from frequency distortion or attenuation of the high and low frequencies across the available 3 kHz bandwidth.
There is commonly phase distortion or a difference in the time delay for each frequency component across the available 3 kHz bandwidth.
Frequently a hetrodyne offset may be encountered which shifts the received component frequencies with respect to the transmitted component frequencies; this results from frequency variations between the telephone company's hetrodyne oscillators.
Amplitude distortion is common and often caused by non-linear amplification of the telephone company's A/D converters.
Impulse noise is common and often results from line "hits" (i.e. lightning) or dial "clicks" from other telephone circuits.
Cross-talk is not a rare occurrence; cross-talk is the "leaking" of voices or tones from one line to another.
Phase jumps (i.e. the instantaneous changes of time delay or phase) of the carrier are common.
Amplitude jumps (i.e. instantaneous changes in amplitude generally as a result of alternate microwave link switch overs) are all too common.
Gaussian noise is an ever present impairment that plagues all electrical systems.
Echo suppressors are installed by the telephone company to permit very long distance voice communication, but they must be disabled for long distance data two way communication.
Typically, high speed modems have suffered from one hybrid loss or the undesired return of a portion of the transmitted signal into the local receiver channel.
Echo on telephone lines is the typical occurrence of return of the transmitted signal back into the receiver channel usually on long distance communications.
Satellite delay is a further line impairment caused by the delay encountered by the distances travelled by telephone signals when beamed to geostable earth satellites.
In addition to the foregoing, several other factors must be kept in mind when considering digital data telecommunications at, say, 9600 bits per second (bps). First, achieving reliable 9600 bps over a dial up line is virtually unheard of. To obtain reliable 9600 bps communication over a telephone line, conditioning is often a necessity; a conditioned line is one of which the user pays a premium and is assured by the telephone company that the line is of lower (or customized) noise characteristics. Conditioning is tailored to the high speed modem manufacturer's specifications, and conditioning cannot obviously be obtained for a dial up line (that would mean conditioning all lines). Second, 9600 bps full duplex operation can only be obtained with modems of the prior art when utilizing two lines (i.e. four wires), certainly not on a single channel and never on dial up lines. Third, modems of the prior art which do manage to achieve 9600 bps on conditioned lines do not gracefully degrade their performance in the presence of impairments. That is, a condititioned line does not guarantee zero impairments; it only statistically reduces the probability of impairments. Yet, when 9600 bps modems of the prior art encounter noise, as they inevitably will, they typically reduce the transmitted data rate to 7200 bps, 4800 bps, 2400 bps, 1200 bps, and so on until reliable communication is re-established. Often, though, impairments of the telephone line are limited to particular frequency bandwidths, and thus, reducing the net data transmission rate usually by a factor of two or four is a needless and uneconomic waste of the available telephone bandwidth.
One high speed digital modem of the prior art is the SM9600 Super Modem manufactured by Gandalf Data, Inc. The SM9600 is nominally a full duplex, 9600 bps modem which can operate over a dial up line. However, the SM9600 data sheets recommend conditioning of the line. In the presence of impairments, the SM9600 will "gearshift" or drop back its transmitted data rate to 4800 bps or 2400 bps. Further, the SM9600 cannot operate full duplex at 9600 bps over a single channel but must allocate a portion of the available spectrum for a reverse channel if a single channel is all that is avialable or utilize a second line (i.e., a total of four wires) for full duplex operation. Further, still, the SM9600 cannot cope with single or multiple extraneous tones within the passband.
The most common class of 9600 bps digital data modems of the prior art is available from AT&T (Model 2096A). This class of modem is extremely sensitive to impulse noise (i.e., dial clicks and lightning strikes). It will tolerate but 5 Hz frequency heterodyne offset, and it requires use of a conditioned line for 9600 bps. Further, this class of modem is extremely sensitive to cross-talk, and the unit does not have any capability for error suppression.
U.S. Pat. No. 3,706,929 to Robinson et al. discloses a combined modem and vocorder pipeline processor. In Robinson, the modem function is implemented using 16 frequency division multiplexed channels, data being carried on each channel by means of phase shift keyed modulation of each carrier. Robinson, however, requires a four wire circuit or a completely separate line for the reverse channel in order to achieve full duplex operation, and conditioning of the line is a necessity to achieve high data rates.
U.S. Pat. No. 4,206,320 to Keasler et al. discloses a high speed modem suitable for operating with a switched network. However, Keasler requires a completely separate reverse channel to obtain full duplex operation. Further, while utilizing 32 carriers to convey the information in a frequency division multiplexed manner, Keasler employs the inefficient mechanism of forming a "hole" or delay at the beginning and end of each modulation sub-period in order to minimize the effects of intersymbol distortion. The shortened modulation period can cause undesired crosstalk between adjacent channels.
Several other aspects tend to render all high speed digital data modems of the prior art, including those referenced hereinabove, obsolete. No modems of the prior art are capable of functioning in both synchronous and asynchronous modes. Synchronous transmission is the old standard form of data communication over a telephone line. However, asynchronous transmission in the form of packetization vastly improves the error performance of the modem. In packet transmission data is formed into blocks (say, from 0 to 256 characters each) which are sent as self contained packets. Each packet contains housekeeping information for framing, routing, error detection, etc. This housekeeping information is placed into packet header and trailing fields. During the last few years there has been an unprecedented, rapid adoption of an international standard for packet switched communication--International Standards Organization X.25. X.25 is a multilevel protocol with only the lower levels which deal with the transmission of the data itself now unambiguously defined. There is a major advantage in having a high speed modem that is capable of X.25 protocol compatibility for obvious internet communication. There is a further significant advantage in having a modem that can communicate over internets utilizing protocols other than X.25 (each computer company devised its own protocol prior to X.25). There are simply no high speed digital data modems of the prior art which are compatibile with packet switched networks (not to mention any that are compatible with multiple packet switching internet protocols) and which operate in both synchronous and asynchonous modes concurrently through signal multiplexing.
In addition to the foregoing, modems of the prior art suffer from the inability to isolate or otherwise pinpoint the source of poor error performance to either the modem or the communication medium. High speed modems of the prior art commonly calculate a composite data error rate which aggregates causes originating in the modem itself and the causes originating from the telephone circuit. Often the most vexing problem to the data communication manager is assigning blame correctly for poor communication performance so that the problem can be fixed. It is usual for the telephone company to immediately deny that any transient fault is due to its equipment. By their nature, transient faults tend to disappear in time and later gross measurements often in fact find nothing wrong with the telephone circuit. Further, even if the circuit impairment persists, measurements made by the telephone company are often incomplete with respect to the range of possible impairments. There are simply no modems of the prior art which can separate modem performance errors from telephone circuit impairments. Moreover, there are simply no modems of the prior art which permit the modem user to completely characterize the telephone circuit so that the user can adequately direct repairs to the telephone circuit by the telephone company.